The invention relates generally to improved semiconductor imaging devices and in particular to a silicon imaging device which can be fabricated using a standard CMOS process. Particularly, the invention relates to CMOS imager having a buried contact at a diffusion node which is electrically connected to a gate of a source follower transistor.
There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. CCDs are often employed for image acquisition and enjoy a number of advantages which makes it the incumbent technology, particularly for small size imaging applications. CCDs are also capable of large formats with small pixel size and they employ low noise charge domain processing techniques. However, CCD imagers also suffer from a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays. Additionally, while offering high performance, CCD arrays are difficult to integrate with CMOS processing in part due to a different processing technology and to their high capacitances, complicating the integration of on-chip drive and signal processing electronics with the CCD array. While there has been some attempts to integrate on-chip signal processing with the CCD array, these attempts have not been entirely successful. CCDs also must transfer an image by line charge transfers from pixel to pixel, requiring that the entire array be read out into a memory before individual pixels or groups of pixels can be accessed and processed. This takes time. CCDs may also suffer from incomplete charge transfer from pixel to pixel during charge transfer which also results in image smear.
Because of the inherent limitations in CCD technology, there is an interest in CMOS imagers for possible use as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.
The advantages of CMOS imagers over CCD imagers are that CMOS imagers have a low voltage operation and low power consumption; CMOS imagers are compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion); CMOS imagers allow random access to the image data; and CMOS imagers have lower fabrication costs as compared with the conventional CCD since standard CMOS processing techniques can be used. Additionally, low power consumption is achieved for CMOS imagers because only one row of pixels at a time needs to be active during the readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photodiode or photoconductor overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, the photodiode or the photoconductor having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate or a photoconductor. For photo diodes, image lag can be eliminated by completely depleting the photodiode upon readout.
CMOS imagers of the type discussed above are generally known as discussed, for example, in Nixon et al., xe2x80x9c256xc3x97256 CMOS Active Pixel Sensor Camera-on-a-Chip,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 31(12) pp. 2046-2050, 1996; Mendis et al, xe2x80x9cCMOS Active Pixel Image Sensors,xe2x80x9d IEEE Transactions on Electron Devices, Vol. 41(3) pp. 452-453, 1994 as well as U.S. Pat. No. 5,708,263 and U.S. Pat. No. 5,471,515, which are herein incorporated by reference.
To provide context for the invention, an exemplary CMOS imaging circuit is described below with reference to FIG. 1. The circuit described below, for example, includes a photogate for accumulating photo-generated charge in an underlying portion of the substrate. It should be understood that the CMOS imager may include a photodiode or other image to charge converting device, in lieu of a photogate, as the initial accumulator for photo-generated charge.
Reference is now made to FIG. 1 which shows a simplified circuit for a pixel of an exemplary CMOS imager using a photogate and having a pixel photodetector circuit 14 and a readout circuit 60. It should be understood that while FIG. 1 shows the circuitry for operation of a single pixel, that in practical use there will be an Mxc3x97N array of pixels arranged in rows and columns with the pixels of the array accessed using row and column select circuitry, as described in more detail below.
The photodetector circuit 14 is shown in part as a cross-sectional view of a semiconductor substrate 16 typically a p-type silicon, having a surface well of p-type material 20. An optional layer 18 of p-type material may be used if desired, but is not required. Substrate 16 may be formed of, for example, Si, SiGe, Ge, and GaAs. Typically the entire substrate 16 is p-type doped silicon substrate and may contain a surface p-well 20 (with layer 18 omitted), but many other options are possible, such as, for example p on pxe2x88x92 substrates, p on p+ substrates, p-wells in n-type substrates or the like. The terms wafer or substrate used in the description includes any semiconductor-based structure having an exposed surface in which to form the circuit structure used in the invention. Wafer and substrate are to be understood as including , silicon-on-insulator (SOI) technology, silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to form regions/junctions in the base semiconductor structure or foundation.
An insulating layer 22 such as, for example, silicon dioxide is formed on the upper surface of p-well 20. The p-type layer may be a p-well formed in substrate 16. A photogate 24 thin enough to pass radiant energy or of a material which passes radiant energy is formed on the insulating layer 22. The photogate 24 receives an applied control signal PG which causes the initial accumulation of pixel charges in n+ region 26. The n+ type region 26, adjacent one side of photogate 24, is formed in the upper surface of p-well 20. A transfer gate 28 is formed on insulating layer 22 between n+ type region 26 and a second n+ type region 30 formed in p-well 20. The n+ regions 26 nd 30 and transfer gate 28 form a charge transfer transistor 29 which is controlled by a transfer signal TX. The n+ region 30 is typically called a floating diffusion region. It is also a node for passing charge accumulated thereat to the gate of a source follower transistor 36 described below. A reset gate 32 is also formed on insulating layer 22 adjacent and between n+ type region 30 and another n+ region 34 which is also formed in p-well 20. The reset gate 32 and n+ regions 30 and 34 form a reset transistor 31 which is controlled by a reset signal RST. The n+ type region 34 is coupled to voltage source VDD. The transfer and reset transistors 29, 31 are n-channel transistors as described in this implementation of a CMOS imager circuit in a p-well. It should be understood that it is possible to implement a CMOS imager in an n-well in which case each of the transistors would be p-channel transistors. It should also be noted that while FIG. 1 shows the use of a transfer gate 28 and associated transistor 29, this structure provides advantages, but is not required.
Photodetector circuit 14 also includes two additional n-channel transistors, source follower transistor 36 and row select transistor 38. Transistors 36, 38 are coupled in series, source to drain, with the source of transistor 36 also coupled over lead 40 to voltage source VDD and the drain of transistor 38 coupled to a lead 42. The drain of row select transistor 38 is connected via conductor 42 to the drains of similar row select transistors for other pixels in a given pixel row. A load transistor 39 is also coupled between the drain of transistor 38 and a voltage source VSS. Transistor 39 is kept on by a signal VLN applied to its gate.
The imager includes a readout circuit 60 which includes a signal sample and hold (S/H) circuit including a S/H n-channel field effect transistor 62 and a signal storage capacitor 64 connected to the source follower transistor 36 through row transistor 38. The other side of the capacitor 64 is connected to a source voltage VSS. The upper side of the capacitor 64 is also connected to the gate of a p-channel output transistor 66. The drain of the output transistor 66 is connected through a column select transistor 68 to a signal sample output node VOUTS and through a load transistor 70 to the voltage supply VDD. A signal called xe2x80x9csignal sample and holdxe2x80x9d (SHS) briefly turns on the S/H transistor 62 after the charge accumulated beneath the photogate electrode 24 has been transferred to the floating diffusion node 30 and from there to the source follower transistor 36 and through row select transistor 38 to line 42, so that the capacitor 64 stores a voltage representing the amount of charge previously accumulated beneath the photogate electrode 24.
The readout circuit 60 also includes a reset sample and hold (S/H) circuit including a S/H transistor 72 and a signal storage capacitor 74 connected through the S/H transistor 72 and through the row select transistor 38 to the source of the source follower transistor 36. The other side of the capacitor 74 is connected to the source voltage VSS. The upper side of the capacitor 74 is also connected to the gate of a p-channel output transistor 76. The drain of the output transistor 76 is connected through a p-channel column select transistor 78 to a reset sample output node VOUTR and through a load transistor 80 to the supply voltage VDD. A signal called xe2x80x9creset sample and holdxe2x80x9d (SHR) briefly turns on the S/H transistor 72 immediately after the reset signal RST has caused reset transistor 31 to turn on and reset the potential of the floating diffusion node 30, so that the capacitor 74 stores the voltage to which the floating diffusion node 30 has been reset.
The readout circuit 60 provides correlated sampling of the potential of the floating diffusion node 30, first of the reset charge applied to node 30 by reset transistor 31 and then of the stored charge from the photogate 24. The two samplings of the diffusion node 30 charges produce respective output voltages VOUTR and VOUTS of the readout circuit 60. These voltages are then subtracted (VOUTSxe2x88x92VOUTR) by subtractor 82 to provide an output signal terminal 81 which is an image signal independent of pixel to pixel variations caused by fabrication variations in the reset voltage transistor 31 which might cause pixel to pixel variations in the output signal.
FIG. 2 illustrates a block diagram for a CMOS imager having a pixel array 200 with each pixel cell being constructed in the manner shown by element 14 of FIG. 1. FIG. 4 shows a 2xc3x972 portion of pixel array 200. Pixel array 200 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 200 are all turned on at the same time by a row select line, e.g., line 86, and the pixels of each column are selectively output by a column select line, e.g., line 42. A plurality of rows and column lines are provided for the entire array 200. The row lines are selectively activated by the row driver 210 in response to row address decoder 220 and the column select lines are selectively activated by the column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel. The CMOS imager is operated by the control circuit 250 which controls address decoders 220, 270 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 210, 260 which apply driving voltage to the drive transistors of the selected row and column lines.
FIG. 3 shows a simplified timing diagram for the signals used to transfer charge out of photodetector circuit 14 of the FIG. 1 CMOS imager. The photogate signal PG is nominally set to 5V and the reset signal RST is nominally set at 2.5V. As can be seen from the figure, the process is begun at time t0 by briefly pulsing reset voltage RST to 5V. The RST voltage, which is applied to the gate 32 of reset transistor 31, causes transistor 31 to turn on and the floating diffusion node 30 to charge to the VDD voltage present at n+ region 34 (less the voltage drop Vth of transistor 31). This resets the floating diffusion node 30 to a predetermined voltage (VDDxe2x88x92Vth). The charge on floating diffusion node 30 is applied to the gate of the source follower transistor 36 to control the current passing through transistor 38, which has been turned on by a row select (ROW) signal, and load transistor 39. This current is translated into a voltage on line 42 which is next sampled by providing a SHR signal to the S/H transistor 72 which charges capacitor 74 with the source follower transistor output voltage on line 42 representing the reset charge present at floating diffusion node 30. The PG signal is next pulsed to 0 volts, causing charge to be collected in n+ region 26. A transfer gate voltage pulse TX, similar to the reset pulse RST, is then applied to transfer gate 28 of transistor 29 to cause the charge in n+ region 26 to transfer to floating diffusion node 30. It should be understood that for the case of a photogate, the transfer gate voltage TX may be pulsed or held to a fixed DC potential. For the implementation of a photodiode with a transfer gate, the transfer gate voltage TX must be pulsed. The new output voltage on line 42 generated by source follower transistor 36 current is then sampled onto capacitor 64 by enabling the sample and hold switch 62 by signal SHS. The column select signal is next applied to transistors 68 and 70 and the respective charges stored in capacitors 64 and 74 are subtracted in subtractor 82 to provide a pixel output signal at terminal 81. It should also be understood that CMOS imagers may dispense with the transistor gate 28 and associated transistor 29 or retain these structures while biasing the transfer transistor gate 28 to an always xe2x80x9conxe2x80x9d state.
The operation of the charge collection of the CMOS imager is known in the art and is described in several publications such as Mendis et al., xe2x80x9cProgress in CMOS Active Pixel Image Sensors,xe2x80x9d SPIE Vol. 2172, pp. 19-29 1994; Mendis et al., xe2x80x9cCMOS Active Pixel Image Sensors for Highly Integrated Imaging Systems,xe2x80x9d IEEE Journal of Solid State Circuits, Vol. 32(2), 1997; and Eric R, Fossum, xe2x80x9cCMOS Image Sensors: Electronic Camera on a Chip,xe2x80x9d IEDM Vol. 95 pages 17-25 (1995) as well as other publications. These references are incorporated herein by reference.
Prior CMOS imagers suffer from several drawbacks regarding the charge flow and contact between the floating diffusion area 30 and the source follower transistor 36. For example, during etching to create the contact between the floating diffusion region 30 and the source follower transistor 36 caution must be taken to avoid over etching into the shallow n-doped region of the floating diffusion region so as to prevent potential charge leakage into the substrate during operation of the imager. Since the size of the pixel electrical signal is very small due to the collection of photons in the photo array, the signal to noise ratio of the pixel should be as high as possible within a pixel. Thus, leakage into the substrate is a significant problem to be avoided in CMOS imagers.
Additionally, the tungsten metal, which is typically used to contact the floating diffusion region and the source follower transistor, is deposited with tungsten fluoride and a reaction sometimes takes place between the tungsten fluoride and the substrate resulting in the formation of silicon fluoride which creates worm holes in the substrate. These worm holes create a conductive channel for current to leak into the substrate, creating a poor performance for the imager. Also, conventional floating diffusion regions typically have a highly n-doped region to facilitate an ohmic metal-semiconductor contact between the contact metallization and the underlying n-doped silicon region to achieve charge transfer to the source follower transistor 36. However, this same highly doped n+ region 30 creates current leakage into the substrate due to high electric fields caused by the abrupt junction. Also, typically there must be an over etch of the contact to account for non-uniformities across the wafer and non-uniformity of the BPSG thickness. Accordingly, resistance in the conductive path between the floating diffusion region and gate of the source follower transistor should be as low as possible without resulting in added junction leakage.
Several of the above-described drawbacks can be seen from FIGS. 5-7 which show a side view of several CMOS imagers of the prior art. It should be understood that similar reference numbers correspond to similar elements for FIGS. 5-7. Reference is now made to FIG. 5. This figure shows the region between the floating diffusion and the source follower transistor of a prior CMOS imager having a photogate as the photoactive area and further includes a transfer gate. The imager 100 is provided with three doped regions 143, 126 and 115, which are doped to a conductivity type different from that of the substrate, for exemplary purposes regions 143, 126 and 115 are treated as n type, which are within a p-well of a substrate. The first doped region 143 is the photosite charge collector, and it underlies a portion of the photogate 142, which is a thin layer of material transparent or partially transparent to radiant energy, such as polysilicon. The first doped region 143 is typically an n-doped region. An insulating layer 140 of silicon dioxide, silicon nitride, or other suitable material is formed over a surface of the doped layer 143 of the substrate between the photogate 142 and first doped region 143.
The second doped region 126 transfers charge collected by the photogate 142 and it serves as the source for the transfer transistor 128. The transfer transistor 128 includes a transfer gate 139 formed over a gate oxide layer 140. The transfer gate 139 has insulating spacers 149 formed on its sides. The third doped region 115 is the floating diffusion region and is connected to a gate 136 of a source follower transistor by contact lines 125, 127, 129 which are typically metal contact lines as described in more detail below. The imager 100 typically includes a highly n+ doped region 120 within n-doped region 115 under the floating diffusion region contact 125 which provides good ohmic contact of the contact 125 with the n-doped region 115. The floating diffusion contact 125 connects n+ region 120 of the floating diffusion region with the gate 136 of the source follower transistor. In other embodiments of the prior art, the entire region 115 may be doped n+ thereby eliminating the need for n+ region 120.
The source and drain regions of the source follower transistor are not seen in FIG. 5 as they are perpendicular to the page but are on either side of gate 136. The source follower gate 136 is usually formed of a doped polysilicon which may be silicided and which is deposited over a gate oxide 140, such as silicon dioxide. The floating diffusion contact 125 is usually formed of a tungsten plug typically a Ti/TiN/W metallization stack as described in further detail below. The floating diffusion contact 125 is formed in an insulating layer 135 which is typically an undoped oxide followed by the deposition of a doped oxide such as a BPSG layer deposited over the substrate. The tungsten metal which forms the floating diffusion/source follower contact 125 is typically deposited using a tungsten fluoride such as WF6.
Typically, the layer 135 must be etched with a selective dry etch process prior to depositing the tungsten plug connector 125. The imager 100 also includes a source follower contact 127 formed in layer 135 in a similar fashion to floating diffusion contact 125. Source follower contact 127 is also usually formed of a tungsten plug typically a Ti/TiN/W metallization stack as described in further detail below. The floating diffusion contact 125 and the source follower contact 127 are connected by a metal layer 129 formed over layer 135. Typically metal layer 129 is formed of aluminum, copper or any other metal.
Separating the source follower transistor gate 136 and the floating diffusion region 115 is a field oxide layer 132, which serves to surround and isolate the cells. The field oxide 132 may be formed by thermal oxidation of the substrate or in the Local Oxidation of Silicon (LOCOS) or by the Shallow Trench Isolation (STI) process which involves the chemical vapor deposition of an oxide material.
It should be understood that while FIG. 5 shows an imager having a photogate as the photoactive area and additionally includes a transfer transistor, additional imager structures are also well known. For example, CMOS imagers having a photodiode or a photoconductor as the photoactive area are known. Additionally, while a transfer transistor has some advantages as described above, it is not required.
The prior art metal contacts 125, 127 described with reference to FIG. 5 typically include a thin layer 123 formed of titanium, titanium nitride or a mixture thereof formed in the etched space in the layer 135. A tungsten plug 122 is then filled in the etched space in the layer 135 inside the thin layer 123. The contact 125 contacts n+ region 120 and forms a TiSi2 area 121 by a reaction between the titanium from layer 123 with the silicon substrate in n+ region 120.
Reference is now made to FIG. 6. This figure illustrates a partially cut away side view of a semiconductor imager undergoing a processing method according to the prior art. The imager 104 has the floating diffusion region 115 having an n+ doped region 120 and the source follower transistor gate 136 already formed therein. The floating diffusion 115 and the source follower gate 136 are under layer 135, which, as noted, is preferably composed of oxides, typically a layered structure of an undoped and doped, i.e., BPSG, oxides. A resist 155 is applied to layer 135 in order to etch through layer 135 to form the contacts to the floating diffusion region 115 and the source follower transistor gate 136. Layer 135 is then etched to form the hole 156 in layer 135 for the floating diffusion contact 125 and hole 157 in layer 135 for the source follower transistor contact 127 as shown in FIG. 7. However, as can be seen from FIG. 7, since the field oxide 132 and layer 135 are both similar oxides it is difficult to control the etching process when attempting to align the hole 156 with the edge of the field oxide 132. In fact, the etching process often etches deep into the n+ region 120 or etches through the exposed edge of the field oxide 132 causing charge leakage to the substrate as shown by the arrows in FIG. 7. Etching deep into the n+ region 120 results in poor contact resistance to the n+ region 120. Etching through the n+ region 120 or through the exposed region of the filed oxide 132 can result in charge leakage to the substrate.
The devices described with reference to FIGS. 5-7 have several drawbacks. For example, during etching, caution must be taken to avoid etching through the n+ layer 120 or even deep into n-doped region 115 where the n-type dopant concentration is reduced. Additionally, when the tungsten metal is deposited by the tungsten fluoride, a reaction sometimes takes place between the tungsten fluoride and the substrate resulting in the formation of silicon fluoride which creates worm holes through the n+ region 120 and into the substrate. These worm holes may create a channel for current to leak into the substrate, creating a poor performance for the imager. While Ti/TiN barrier layers are deposited to: form a good ohmic contact to the n+ region due to the TiSi2 reaction and provide a TiN barrier between the W metallization and the Si substrate, worm holes and contact leakage still occur. Also, the prior art floating diffusion region 115 included the highly n+ region 120 to provide an ohmic contact; however, this same highly doped n+ region sets up high electric fields with respect to the p-type region under field oxide region 132 which fosters current leakage into the substrate. Accordingly, a better low resistance conductive path is required between region 120 and gate 136 of the source follower transistor which provides a good ohmic contact, while avoiding substrate leakage.
The present invention provides a CMOS imager in which the floating diffusion is connected to a gate of the source follower transistor through a buried contact. In a preferred implementation, the buried contact between the floating diffusion and the gate of the source follower transistor is formed on the substrate and under the BPSG layer to reduce the need to etch holes in the BPSG layer. The buried contact provides a better ohmhic contact with less leakage into the substrate. The buried contact between the floating diffusion region and the gate of the source follower transistor also allows the floating diffusion region and the source follower transistor to be placed closer together, thereby reducing size of a pixel and allowing an increased photoarea per cell size which, it turn, increases the signal, to noise ratio of the imager. In addition, the problems with worm holes and connecting of the floating diffusion contact are completely avoided as there is no need for the highly doped n+ region 120 in the present invention and additionally no need for any metallization to be directly in contact with the silicon substrate at the floating diffusion node.